The present invention relates to a process for bending a measuring tube for a Coriolis mass flow rate sensor into a desired shape by means of a tube section of a predetermined length and by means of a two-piece press mold, which is matched to both the measuring tube, to the desired shape, and which has an inner diameter and an outer diameter.
Such mass flow rate sensors can be divided into two classes, based on the shape of the measuring tube(s), namely mass flow rate sensors with at least one straight measuring tube, and those with at least one bent measuring tube. In connection with mass flow rate sensors with bent measuring tubes, those are of interest for the present invention, whose measuring tubes are bent in one plane, i.e. wherein the respective axis of the measuring tubes lie in this plane.
Such measuring tubes are bent, for example, into a U-shape (see U.S. Pat. No. 5,394,758), or are only bent very flat (see U.S. Pat. No. 5,796,011), or are bent into a V-shape in accordance with provisional application No. 60/205 983, filed May 19, 2000.
Such measuring tubes, which are bent in one plane and which have an inner and an outer diameter, can be bent into the desired shape by means of a two-piece press mold, which has been matched to the desired shape.
Tubes to be bent, which in most cases must be heated for this purpose, customarily must be filled with a filler material, which is as incompressible as possible and which assures, that the shape of the cross section, in most cases a circular cross section, has remained as unchanged as possible after the bending process.
It has been shown that, when bending measuring tubes for Coriolis mass flow rate sensors, this customary method using only a single incompressible filler material is not sufficient for achieving the high mechanical-geometric accuracy of the bend and of the bent measuring tubes.
Moreover, for reasons of efficiency of production it is absolutely required that the curved measuring tubes no longer need to be worked with respect to their length, in particular to be shortened. For a production batch it should therefore be possible to firmly preset the length of the straight, not yet bent tube, so that the bent measuring tubes all have a uniform, appropriately shortened final length.
To solve this problem, the object of the present invention therefore lies in providing a process for bending a measuring tube for a Coriolis mass flow rate sensor into a desired shape from a tube section of a preset length and by means of a two-piece press mold, which is matched to both the measuring tube having an inner diameter and an outer diameter, and to the desired shape.
This object is attained by means of the following steps:
inserting a flexible support body into the tube section such that one end of the tube section is closed, the maximum outer diameter of the flexible support body being less than the inner diameter of the tube section;
filling the tube section with the support body therein with a liquid;
permitting the liquid to solidify completely;
providing a two-piece press mold, which is matched to the outer diameter of the tube section and to the desired shape of tube section;
placing the tube section filled with the support body and the solidified liquid into the opened press mold;
closing the press mold thereby bending the tube section into the desired shape;
opening the press mold and removing the bent tube section; and
permitting the solidified liquid to melt and thereafter removing the support body from the bent tube section along with the melted liquid.
In accordance with a first preferred embodiment of the present invention, a liquid is used which solidifies at a temperature of less than 280xc2x0 C.
In accordance with a first preferred further embodiment thereof, molten bismuth is used, in a second further preferred embodiment the liquid solidifies at a temperature of less than 50xc2x0 C.
In further preferred embodiments thereof, Wood""s alloy, or an aqueous solution, in particular water itself, or waxes or oils are used.
In accordance with a second preferred embodiment of the present invention, a helical spring is used as the support body, which is preferably centered in the tube section by means of spacer rings. In both cases a helical spring with windings which rest against each other is preferably used.
It has been shown in a surprising manner that the combined use of a liquid together with the support body makes possible a highly accurate bending of measuring tubes of Coriolis mass flow rate sensors.
Since the support body in accordance with a characteristic of the present invention has a maximum diameter which is less than the inner diameter of the measuring tube, the support body can be easily pulled out of the measuring tube again after the bending operation and after the solidified liquid has again become liquid.
It is believed that the reason for the great accuracy during bending, i.e. the completely unaffected cross section of the measuring tube in the bent area, is that, although the solidified liquid as the filler material contributes the greater part to this high accuracy, the support body has a thin layer of the solidified liquid, which is located between the inner wall of the measuring tube and the support body and contributes the remaining portion to the high accuracy.
The present invention will now be explained in greater detail by means of the drawing figures, in which successive steps of a process in accordance with the present invention are represented- Parts with the same functions in the different figures have been provided with the same reference numerals, but are only repeated in subsequent figures if this appears to be useful.